https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101621
Bug ID: 101621
Summary: gcc cannot optimize int8_t vector assign with
subscription to shuffle
Product: gcc
Version: 11.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: yumeyao at gmail dot com
Target Milestone: ---
https://gcc.godbolt.org/z/91cqenf99
typedef char v16b __attribute__((vector_size(16)));
To summary it up, regarding optimizing v = { v[n] ...} into shuffle, targeting
Intel x86(x86_64):
These is a lack of optimization when there is a zero
There is some regression starting from gcc9.
so this might be 2 issues. But I think a proper fix could resolve both.
* gcc can optimize int8_t vector assign with subscription of the same vector to
shuffle, like this:
v16b gcc_can_shuffle(v16b b) {
return (v16b) {b[0], b[0], b[0], b[0], b[4], b[4], b[4], b[4], b[8], b[8],
b[8], b[8], b[12], b[12], b[12], b[12]};
}
* However, if there is a zero, gcc can't handle this. Actually this is
supported on Intel x86, with a negative subscription indicating the 'zero
value'.
Clang can do the optimization starting with clang 5.
* Furthermore, there is a regression:
gcc < 8 can always optimize it, but starting with gcc9, if there is a cast,
then the optimization fails:
typedef long v2si64 __attribute__((vector_size(16)));
v16b gcc_cannot_shuffle_with_cast(v2si64 x) {
v16b b = (v16b)x;
v16b b0 = {b[0], b[0], b[0], b[0], b[4], b[4], b[4], b[4], b[8], b[8],
b[8], b[8], b[12], b[12], b[12], b[12]};
return b0;
}
gcc 11 can optimize it on -O3, but not on -O1 or -O2.
